The hydrogenation of carbonyl compounds, in particular of carboxylic acids or carboxylic esters, with the aid of heterogeneous catalysts plays an important role in the chemical industry. In principle, both slurry applications and fixed-bed applications are possible for these hydrogenations, with fixed-bed applications predominating. In the slurry process, the catalysts are used as a powder, whereas shaped catalyst bodies are used in the fixed-bed processes.
Ni—, Cu—, Co— or noble metal-containing catalysts, in particular, are used for the hydrogenation of carbonyl compounds. These can be used as all-active catalysts (e.g. Raney catalysts) or as supported catalysts.
The patent documents DE 43 45 265 and DE 43 35 360 describe shaped Raney catalysts based on Ni, Co, Cu and Fe. These are used for the hydrogenation of organic compounds. A disadvantage of these catalysts results from the addition of metal powder as binder, with the added metal powder having a lower catalytic activity than the Raney metal.
The production of shaped Raney catalysts without addition of binders is described in EP 880 996. These catalysts are used for the hydrogenation of nitriles. To produce these catalysts, a metal-aluminum alloy present as powder is mixed with a high molecular weight polymer and optionally promoters and subsequently is shaped to give shaped bodies. The shaped bodies are calcined at temperatures of up to 850° C., which leads to controlled decomposition of the polymer and formation of a fixed-bed catalyst having satisfactory mechanical stability. Activation is effected by leaching out the aluminum using sodium hydroxide solution. However, the leaching out of the aluminum and thus activation of the catalyst occurs merely in the outer shell of the shaped body. The core of the catalyst continues to consist of the metal-aluminum alloy used and serves as support for the activated outer layer of the catalyst. As a result, a considerable proportion of the relatively expensive alloys remains unused.
The hydrogenation of carbonyl compounds is carried out using not only the Raney catalysts but also essentially Cu and Ni catalysts supported on various metal oxides such as Al2O3 or SiO2.
Thus, for example, U.S. Pat. No. 4,666,879 describes an extruded copper chromite-aluminum oxide catalyst which is produced by mixing from 40 to 82% by weight of copper chromite and from 18 to 60% by weight of aluminum oxide. The Al2O3 is typically used in the form of pseudoboehmite or hydroxyboehmite. The extruded catalyst is, after calcination, suitable for the liquid- and gas-phase hydrogenation and hydrogenolysis of various carbonyl compounds and functional side groups of aromatic compounds. The BET surface area of the extruded catalyst is typically in the range from 125 to 225 m2/g.
U.S. Pat. No. 4,762,817 describes a catalyst for the hydrogenation of aldehydes, which consist essentially of a mixture of copper and zinc oxide. An improvement in the selectivity was able to be achieved by impregnation with alkali metals such as sodium, potassium, lithium or cesium in combination with a transition metal such as nickel, cobalt or mixtures thereof.
The U.S. Pat. No. 4,929,771 describes catalyst compositions containing oxides of Cu and Ti and the use of such catalyst compositions in the hydrogenation of particular esters to form the corresponding alcohols.
The U.S. Pat. No. 5,008,235 describes a process for the hydrogenation of organic aromatic or nonaromatic acids and esters thereof to form the corresponding alcohols using a coprecipitated catalyst. The catalyst contains copper, aluminum and a further metal such as magnesium, zinc, titanium, zirconium, tin, nickel, cobalt or mixtures thereof and is subjected to reduction before use. The temperature in the reduction is increased stepwise to a final temperature of from 150° C. to 250° C.
The U.S. Pat. No. 5,093,534 describes a two-stage process for the hydrogenation of saturated and unsaturated aldehydes to alcohols using Cu— and Ni— containing catalysts. The first stage of the hydrogenation is carried out using a particulate alkaline copper catalyst. In the second stage of the hydrogenation, a supported nickel-containing catalyst whose support material has acidic sites having a particular acid strength is used.
The U.S. Pat. No. 5,124,295 describes an extruded copper chromite catalyst consisting of a mixture containing from about 20 to 80% by weight of copper chromite and from about 20 to 80% by weight of an extrudable inorganic binder. The catalyst has a specific surface area of from about 20 to 225 m2/g, and the total pore volume of the pores in the catalyst is from 0.35 to 1 cm3/g. In one embodiment, this document describes a process for producing a shaped copper chromite catalyst by production of an extrudable mixture, extrusion of the mixture and calcination of the extrudate. The catalysts are employed for the hydrogenation of aldehydes, ketones, carboxylic acids and carboxylic esters.
The U.S. Pat. No. 5,134,108 describes a hydrogenation catalyst comprising oxides of a first metal, copper or zinc, and a second metal, chromium, molybdenum, tungsten or vanadium, and optionally an oxide of a promoter such as manganese, barium, zinc, nickel, cobalt, cadmium or iron. The hydrogenation catalyst is in the form of a powder whose average particle diameter is from about 6 to 20 μm and whose surface area is from about 20 to 70 m2/g. The catalysts are produced by precipitation of the metal salts by means of a base.
U.S. Pat. Nos. 5,155,086 and 5,345,005 describe a pulverulent catalyst which consists largely of the oxides of copper and zinc and a smaller proportion of aluminum oxide, where the atomic ratio of copper to zinc is from 0.2 to 5.5. The catalyst is produced by precipitation, e.g. at a pH of >7, and calcination of the precipitate. The hydrogenation catalysts are used for the hydrogenation of aldehydes, ketones, carboxylic acids and carboxylic esters.
WO 92/04119 describes copper-manganese catalysts for the hydrogenation of fatty acids and esters thereof. They are produced by admixing an aqueous solution of Cu(II) and Mn(II) salts with sodium hydroxide solution, forming a precipitate of Cu hydroxide and Mn hydroxide. This precipitate is then calcined as powder or in tableted form. The catalysts obtained have a BET surface area of from about 3 to 45 m2/g.
WO 02/47818 describes Cu oxide-containing catalysts for the hydrogenation of maleic anhydride and derivatives thereof. As pore formers, use is made here of, in particular, graphite and ammonium nitrate which are mixed into the catalyst powder before tableting. The catalysts in the production of which exclusively graphite was used as pore former had a pore volume of less than 0.2 cm3/g.
WO 97/34694 describes copper oxide/aluminum oxide hydrogenation catalysts produced by precipitation of aqueous solutions of copper nitrate and sodium aluminates using sodium carbonate. The material obtained is, after drying, calcined at from about 400° C. to 700° C. and subsequently tableted with addition of graphite. The pellets have a pore volume of from 0.2 to 0.6 ml/g and a bimodal pore radius distribution having a first maximum at about 10 nm and a second maximum at from about 50 to not more than 200 nm.
In the commercial use of catalysts, an increase in the conversion to the target product, in particular, is of greatest interest in the context of a further improvement in the economics. Pellet-shaped catalysts are predominantly used for hydrogenation reactions. These usually have a higher mechanical stability than, for example, extrudates. The stability is a consequence of the process of tableting since relatively high pressures normally prevail there. However, the pore volume and thus the accessibility of the active sites is frequently reduced by the strong compression during tableting, which has an adverse effect on the conversion in catalytic reactions. To compensate for this disadvantage, pore formers are frequently added during the production of tableted catalysts in the prior art in order to achieve an increase in the pore volume. However, the addition of pore formers incurs the risk that the active catalyst material will be adversely affected by additives. Particularly when using organic pore formers which are burnt out from the catalyst in order to increase the pore volume, carbonization of the surface can take place. In addition, additives, whether organic or inorganic, always involve the risk that impurities and catalysts poisons will be introduced into the catalyst. A further disadvantage of burn-out materials as pore formers is the often very locally arising evolution of heat during burning-out. The active sites are often adversely affected thereby. In particular, sintering effects and a decrease in the dispersion of metal particles and thus a reduction in the activity of the catalysts can occur.
In view of this background, it was an object of the present invention to provide a process for producing tableted catalysts having an increased pore volume, without any pore formers being added.
A further object was to provide tableted catalysts which have a higher activity in hydrogenation reactions compared to catalysts of the prior art.
This object is achieved by the process of the invention and the catalysts obtainable thereby.